1. Field of the Invention
The present invention is directed to a method for the electrophotographic printing of a print image having a plurality of colors on a carrier.
2. Description of the Related Art
U.S. Pat. No. 4,078,929 discloses a method for electrophotographic printing with two colors. This method also bears the name "Tri-Level Method". In this method, a print image contains at least one first image element of a first color and at least one third image element with a third color. A second image element of the print image has a background color of the image carrier, so that no toner particles are to be applied.
The first image element is allocated to a first surface element of a photo conductor layer. The photo conductor layer and an electrode layer carrying a predetermined reference potential are contained in a light-sensitive layer system. The reference potential is usually zero potential. Likewise, the second image element is allocated to a second surface element or region and the third image element is allocated to a third surface element of the photo conductor layer. What is disadvantageous in the tri-level method is that only toner particles of two colors can be applied in a print image. A color mixing with the assistance of three primary colors, accordingly, is precluded without repeating the process. A plurality of printing events are possible for color mixing, whereby a printing event contains the steps of charging, image-wise illumination and developing.
European Published Application EP 0 606 141 A2 discloses a printer that can be employed for the electrophotographic printing of a print image with a number of colors. The maximum plurality of colors with one illumination step, however, is limited to three colors. For example, it is not possible to print additional, decorative colors such as, for example, gold or silver with exact registration in addition to the three primary colors. Given a printer according to the European Published Application, moreover, a layer system that is composed of four layers is employed. Added outlay is needed in order to manufacture this layer system since four layers instead of two layers are to be arranged on top of one another.
Further, two light-sensitive layers contained in the layer system must be illuminated with two light beams that respectively have different wavelengths. In a first version of the print according to the European Published Application, the photoconductor is conducted past an illumination station for the image-wise illuminating twice. As a result thereof, the printing speed is reduced to half. An illumination station must offer two different light beams in both passes. The outlay for the illumination is therefore doubled, for example in view of generating the illumination signals and in view of the demands made of the optical system with respect to the imaging errors.
In a second version of the European Published Application, the light-sensitive layer system is conducted past two illumination locations in one pass. However, two illumination beams must still be offered at each of these illumination locations. The demands made of the imaging precision of the optical system employed are more stringent in the second version. Thus, TE light waves (transversal-electromagnetic) are employed for the first illumination station and TM light waves (transversal-magnetic) are employed in the second illumination station for the different wavelengths of the two illuminating beams.
U.S. Pat. No. 5,155,541 discloses a printing method wherein the tri-level method has been further-developed for printing at least three colors.